A TAB system using a lead finger projectingly formed has been widely known as a method of mounting IC bare chips on a flexible circuit board. This system is chiefly employed for mounting ICs on a circuit board of a single layer. In the case of a mounting system using a multilayer flexible circuit board, as shown in FIG. 12, a flexible multilayer circuit board 1 for the flip-chip system is fabricated by laminating inner and outer electrically conductive layers 2 to 7 for forming required circuit wiring patterns via suitable insulating base members, and an IC bare chip 9 is connected to connection ends 2A and 3A of the outer conductive layers 2 and 3 via bumps 9A and 9B. In the case of FIG. 9, the outer conductive layer 2 is not electrically connected to the inner conductive layer 4, but is connected to the outer conductive layer 6 via a through-hole conductive portion 8. Further, the outer conductive layer 3 is electrically connected to the inner conductive layer 5 and the conductive layer 7 via a through-hole conductive portion 8A. The through-hole conductive portions 8 and 8A are formed suitably away from the bumps 9A and 9B for connection of the IC bare chip 9 and the connection terminals 2A and 3A. The connection ends 2A and 3A of the circuit board 1 and the connection bumps 9A and 9B for the IC bare chip 9 are connected together generally by soldering. Even in the case of the flexible multilayer circuit board 1 of the flip-chip structure, therefore, solder-flow blocking dams 10 and 10A are arranged around the connection ends 2A and 3A.
In the structure of the flexible multilayer circuit board 1 made under the above-mentioned flip-chip system, it is difficult to arrange the through-hole conductive portions 8 and 8A that connect the inner conductive layer to the outer conductive layers in the portion where the IC bare chip 9 is mounted due to the conduction structure using through-holes, and further, it is necessary to provide the solder-flow blocking dams 10 and 10A, putting great limitation on construction of the portion where the IC-bearing chip 9 is mounted on the circuit board 1. Therefore, circuit boards 1 of this type are usually bulky compared to the mounting density of the IC bare chips 9. Furthermore, the connection bumps 9A and 9B must be provided on the IC bare chip 9, requiring an increased number of steps. The increased steps and the provision of the solder-flow blocking dams inevitably raises the manufacturing costs on this IC mounting system.
To form through-hole conductive portions 8 and 8A by a conventional method, on the other hand, it is necessary to bore holes at required conductive portions using a drill, causing a problem of undesirable smear due to the frictional heat produced by the drilling using a drill that revolves at high speeds and a problem of an additional process of removing the smear. Furthermore, when the flexible circuit board of this type is of a soft material, the diameters of the holes is limited. In laminating the layers, moreover, it is not easy to regist the land positions and perforation positions of the required layer.